1. Field of the Invention
The present invention relates to a variable frequency oscillator, for example, used in a phase synchronization circuit and a phase locked loop (PLL) circuit using the same.
2. Description of the Related Art
FIG. 1 is a view of an example of the general configuration of a laser beam printer.
In the laser beam printer 1, normally a raster output scanning method is adopted. A laser beam LO from a laser source 2 is scanned over a predetermined range and converged on a photosensitive drum 4 by a polygon mirror 3 rotating at a predetermined speed. This exposes a charge portion of the photosensitive drum 4 to record an electrostatic latent image on the photosensitive drum 4 which is then printed on paper.
FIG. 2 is a schematic view of the path of a laser beam as it is reflected by the polygon mirror and scans the circumference. In FIG. 2, a solid line shown by A indicates transfer of an image to a plane, while a curve shown by B indicates a constant speed scanning path, respectively.
In the laser beam printer 1, when the laser beam LO is scanned and converged as it is on the plane, that is, the photosensitive drum 4, by a polygon mirror 3 rotating at a predetermined speed, since the rotation speed of the polygon mirror is constant, the scanning speed of the beam passing through the photosensitive drum 4 and a frequency ratio for controlling dots forming one pixel on one line do not become constant and distortion is caused on the print.
In order to prevent distortion, the laser beam printer 1 in FIG. 1 is configured using an fxcex8 lens 5 etc. on an optical path from the polygon mirror 3 to the photosensitive drum 4 and using this optical system to make the frequency ratio for controlling the scanning speed and dots of the beam converged on the photosensitive drum 4 constant.
A laser beam printer having the above configuration, however, requires fine adjustment of the laser beam beyond the precision of correction by an optical system using a fxcex8 lens. This is achieved by precise control for finely adjusting the clock frequency in a PLL circuit generating a clock.
FIG. 3 is a block diagram of an example of the configuration of a general PLL circuit, for example, used in a clock control system of the laser source 2 of the laser beam printer 1 in FIG. 1. Also, FIG. 4A is a timing chart of a reference signal given to the PLL circuit, while FIG. 4B is a timing chart of an output clock signal of the PLL circuit.
The PLL circuit 6 is, as shown in FIG. 3, comprised of a negative feedback circuit including a phase detector (PD) 61, a loop filter (LPF) 62, a voltage-controlled oscillator (VCO) 63, and a programmable counter (PC) 64.
The phase detector 61 compares phases of a reference signal Sr and a divided signal Sdv of the programmable counter 64 and outputs a signal S61 in accordance with the difference to the loop filter 62.
The loop filter 62 generates a control voltage Vc of the VCO 63 in accordance with the signal S61 from the phase detector 61 and outputs it to the VCO 63.
The VCO 63 oscillates at a frequency in accordance with a control voltage Vc by the loop filter 62, generates a clock signal CLKop having a frequency of fop, and outputs it to the laser source and programmable counter 64.
The programmable counter 64 divides the frequency fop of the output clock signal CLKop of the VCO 63 exactly by N (here, N=24) and outputs a divided signal Sdv having a frequency of fop/N to the phase detector 61.
In a PLL circuit 6 having such a configuration, as shown in FIG. 4A and FIG. 4B, since a frequency fr of the reference signal Sr is fixed, an output clock CLKop is generated in synchronization with the reference signal Sr at a constant frequency fop as calculated by fop=Nfr and supplied to the laser source 2.
However, even a laser beam printer 1 using the above PLL circuit 6 cannot prevent deviation of position of the optical system shown in FIG. 5.
Particularly, in color printing, the same positions are scanned by laser beams for the different colors, so error in the correction precision of the optical system appears directly as color deviation.
An object of the present invention is to provide an oscillator having a modulation function able to control the frequency in addition to a control signal and a PLL circuit using the same.
To attain the above object, according to a first aspect of the present invention, there is provided an oscillator for oscillating at a frequency based on a control signal, comprising a modulation means for modulating a frequency by adding a modulation signal to the control signal.
Preferably, a modulation rate of the modulation means is controlled by a ratio of an oscillation frequency to the modulation signal value.
More preferably, the modulation signal value is given digitally.
According to a second aspect of the present invention, there is provided a ring-type oscillator including a plurality of delay stages controlled in delay value by an inverter or buffer and a control signal connected in cascade and forming a closed loop by an inverted phase, comprising a modulation means for modulating an oscillation frequency by adding a modulation signal to the control signal in part of the plurality of delay stages.
Preferably, the modulation rate of the modulation means is controlled by a ratio of an oscillation frequency to the modulation signal value.
More preferably, the modulation signal value makes the control signal value 1/n and furthermore is weighted so as to have a variable range of m bits.
According to a third aspect of the present invention, there is provided a PLL circuit comprising a phase detector for comparing phases of a feedback signal of an output signal and a reference signal and outputting a signal indicating a phase difference; a loop filter for receiving an output signal of the phase detector and outputting a control signal for canceling out the phase difference; and an oscillator for oscillating at a frequency based on a control signal from the loop filter; the oscillator comprising a modulation means for modulating a frequency by adding a modulation signal to the control signal.
Preferably, in the oscillator, a second control signal is controlled by a control signal of the loop filter and the frequency is controlled by the second control signal.
According to a fourth aspect of the present invention, there is provided a PLL circuit comprising a phase detector for comparing phases of a feedback signal of an output signal and a reference signal and outputting a signal indicating a phase difference; a loop filter for receiving an output signal of the phase detector and outputting a control signal for canceling out the phase difference; and an oscillator for oscillating at a frequency based on a control signal from the loop filter; the oscillator forming a ring-type including a plurality of delay stages controlled in delay value by an inverter or buffer and a control signal connected in cascade and forming a closed loop by an inverted phase and comprising a modulation means for modulating an oscillation frequency by adding a modulation signal to the control signal in part of the plurality of delay stages.
According to the present invention, the multiplication clock synchronized with the reference synchronization signal and obtained by the PLL circuit is not modulated in frequency in its synchronization signal.
For example, the oscillator in the PLL circuit modulates the frequency by, for example, a modulation rate given as a digital value, oscillates at a modulated frequency corresponding to a control voltage Vc of a loop filter, and thereby produces a clock signal having a modulated frequency. The clock signal is supplied to a light source of a raster output scanning system.